DE3873201T2 - METHOD FOR MAKING SKIN, LEATHER AND THIS FIBROUS SUBSTRATE. - Google Patents

Abstract

For upgrading split leather and the like, an aqueous emulsion is used of a polyurethane obtained from an isocyanate prepolymer having a molecular weight within the range of 1500 to 3200.

Description

The invention relates to a method for dressing hides, leather and similar fibrous substrates, which is particularly, but not exclusively, suitable as a method for upgrading split leather by means of layer-forming emulsions of suitable synthetic resins, which are applied according to the so-called transfer process using a supporting web of release paper to which a thin layer of an aqueous polyurethane emulsion has been applied, the polyurethane of this emulsion being obtained from an isocyanate prepolymer having a glycol basic unit.

In the field of skin dressing and in particular the upgrading of split leather, various upgrading processes have long been proposed which use aqueous upgrading agents, in particular aqueous polyurethane emulsions, in order to eliminate or at least mitigate the serious environmental pollution problem posed by the organic solvents conventionally used to facilitate the handling of these upgrading agents. Examples of suitable polyurethane emulsions are given in US 4 501 852, EP 0 104 377 and EP 0 193 808.

Experimental work carried out shows that in order to obtain a suitable product, i.e. a product whose aesthetics and strength properties are comparable to those of naturally grained hide, the thickness of the enhancement layer should preferably be less than 0.1 mm. At greater thicknesses, especially in the range of 0.2-0.4 mm, as conventionally provided, it is well known that an external appearance, a feel and a surface structure similar to those of plastic can be obtained.

Obtaining refining layers of such a small thickness as indicated above involves considerable technical problems relating to the application of the polyurethane emulsion to a web of release paper, the bonding to the split leather material to be refinished and the drying processes and times, particularly when the refining process is carried out continuously.

In fact, during various attempts to implement the upgrading processes considered here, it was almost always observed that the final product had significant and unacceptable "discontinuities" in its upgrading layer, which often extended into cracks across its surface. The final product obtained by such attempted upgrading of split leather with aqueous polyurethane emulsions also almost always showed low strength on the flexometer.

To these listed disadvantages, one should add a not insignificant disadvantage, which results from the low throughput of the equipment available for implementing upgrading processes. In fact, equipment used today does not allow a higher feed rate of the release paper web than about 3-4 m/min, and the hourly throughput of the entire system is obviously dependent on this feed rate. Attempts to improve the throughput by increasing the feed rate of the release paper web have always led to inferior results compared to the already unsatisfactory results of low-speed processes. A considerable increase in pockmarks and cracks in the surface layer of the upgrading layer has even been observed.

All of the above-mentioned disadvantages are principally due to a tough "skin" that forms very quickly over the thin layer of aqueous polyurethane emulsion when applied to the release paper web, and this "skin" unfortunately forms an effective barrier against the removal of water contained in the emulsion by evaporation as the release paper web passes through the drying ovens. However, the steam thus released can "break through" the barrier, which in many cases leads to the aforementioned pockmarking and cracking. It is understandable that this disadvantageous phenomenon is more pronounced the higher the rate of evaporation of water from the polyurethane emulsion layer used. Consequently, a low rate of advance of the release paper web through drying ovens of considerable length in which drying can be carried out under moderate temperature conditions should lead to improved properties of the resulting upgraded product. However, attempts made in this direction have, on the one hand, led to products with still unsatisfactory aesthetic properties and, on the other hand, required the availability of large facilities, particularly with regard to the drying ovens.

The object of the present invention is to provide a process for dressing hides, leather and similar fibrous substrates, in particular for upgrading split leather, in which aqueous polyurethane emulsions can be used as the upgrading agents and which yields end products which are advantageously comparable to naturally grained hides, and at high hourly throughputs, in order to thereby overcome all the disadvantages mentioned above in connection with the prior art.

This object is achieved according to the invention by a process of the type mentioned at the outset, which is characterized in that the isocyanate prepolymer has a number-average molecular weight in the range of 1500-3200 and that the glycol basic unit contains at least one acidic polar group.

The isocyanate prepolymer is obtained from a diisocyanate monomer having the formula ONC-R'-NCO, where R is selected from a group comprising hexamethylene, isophorone, methylenedicyclohexyl and a polyether monomer selected from a group comprising:

wherein is in the range of 10-80 and is a glycol monomer containing at least one acidic polar group, preferably a carboxyl group of the formula

where n and m are in the range 1-2 and p is in the range 0-1.

If the polyether is a polypropylene glycol of the formula

is preferably between 16 and 46, in particular between 27 and 40.

If the polyether is a polyethylene glycol with the formula H-(O-CH₂-CH₂)-OH, it is in the range of 25-80, even better in the range of 40-65.

When the polyether is a polybutylene glycol of the formula H-(OCH₂-CH₂-CH₂-CH₂)-OH, is in the range of 12-42, more preferably in the range of 22-35.

The presence of glycol with acidic polar groups, especially and preferably carboxyl glycol in the isocyanate prepolymer is necessary to impart stability to the aqueous emulsion to be produced.

The amount of this carboxyl glycol used for the aforementioned purpose is in the range of 3-15 wt.%, based on the weight of polyether, preferably between 5 and 10 wt.%.

To obtain different strengths, 5% to 30% (wt% based on the polyethers) polyester diols should be selected from those of the following formulas:

H-[O-(CH₂)₄-O-CO-(CH₂)₄-CO] -OH

which is in the range of 7-15;

H-[O-CH-CH₂-O-CO-(CH₂)₄-CO] -OH

which is in the range of 8-16;

CH3;

H-[-O-(CH₂)₆-O-CO-(CH₂)₄-CO] -OH

which is in the range of 6-14.

The aqueous polyurethane emulsion suitable for use in the present invention is obtained from the isocyanate prepolymer by slowly adding this prepolymer to an aqueous solution containing a tertiary amine which does not react with the isocyanate groups and is selected from the amines of the formulas: Triethylamine and where n is in the range 1-2 and m, m' are in the range 0-1.

The amount of tertiary amine to be dissolved in water is a function of the amount of carboxyl glycol contained in the isocyanate prepolymer. The molar ratio of the carboxyl glycol contained in the prepolymer to be emulsified to the amine can be:

Mole tertiary amine/mole carboxyl glycol = x where x is the range 0.7-2.0, preferably 1.0-1.7.

The water may optionally contain small amounts (0.3-1.5 wt% based on the weight of water) of some non-ionic dispersants to make the emulsion more stable.

The type of dispersant used is not specific and does not affect the application characteristics of the final emulsion during the application step, provided that the amounts used do not significantly exceed the recommended percentages.

The prepolymer is added to the solution with vigorous stirring. The water temperature should be in the range of 10-20ºC and the temperature of the prepolymer between 40 and 70ºC.

The viscosity of the resulting solution depends on the amine used as well as on the proportion of carboxyl glycol in the prepolymer and can be adjusted by changing these two values within the specified limits.

After the prepolymer has been emulsion-formed, it should be extended by adding aliphatic diamines of the following formulas:

H₂N-(CH₂)n-NH₂ where n is between 0 and 6; 2,2,4-trimethylhexamethylenediamine; 2,4,4-trimethylhexamethylenediamine; Isophoronediamine

in equimolar amounts with respect to the NCO groups in the prepolymer or by any other comparable amines.

The isocyanate prepolymer reacts with the diamine to form the final polyurethane.

The concentrations of polyurethane in the final emulsion can be between 25 and 50 wt.%.

A few minutes after the addition of the diamine, the polymerization is complete and the polyurethane obtained has a high molecular weight and is suitable for the intended application.

By using an aqueous polyurethane emulsion according to the invention, it was unexpectedly found that a skin was formed over its thin layer, which was coated on the release paper, which allowed the steam generated in the layer itself when heat was applied to "sweat out". This particular unexpected feature was found to prevent the formation of pockmarks and cracks in the layer, even when the heat level during the drying step of that layer was greatly increased in order to increase the hourly throughput, while at the same time the size of the dryer remained unchanged.

This surprising and advantageous feature was then further improved by emulsifying the polyurethane according to the invention in a water/glycol ether azeotrope in which the glycol ether present was 5-13% by weight based on the weight of water used, which allows a more gradual evaporation of the water even at higher drying temperatures.

Such solvents may be selected from propylene glycol monomethyl ether and dipropylene glycol monomethyl ether and the ethylene glycol ethers listed in the following table. TABLE Glycol ether Boiling point [ºC at 1013 mbar (760 mmHG)] Evaporation rate (diethyl ether = 1) Flash point (ºC, open cup) Relative density (25/25 ºC) PM = Propylene glycol monomethyl ether EM = Ethylene glycol monomethyl ether EE = Ethylene glycol monoethyl ether DPM = Dipropylene glycol monomethyl ether

The temperatures at which the thin (0.05-0.1 mm) layer of the aqueous polyurethane emulsion could be dried according to the invention allow a feed rate of up to 8-10 m/min for the release paper web, whereby none of the disadvantages present in the prior art appear in the upgraded product.

Further features and advantages emerge from the following description of some examples of the preparation of the aqueous polyurethane emulsions according to the invention and of a process based on the use of such emulsions. The attached drawings show schematically and by way of example a device for carrying out the process according to the invention.

example 1

Polyester (A) = polybutylene glycol adipate H-[O-(CH₂)₄O-CO-(CH₂)₄-CO] -OH with number average molecular weight = 2032 ( = 10.07)

Polyether (B) = polypropylene glycol

with number average molecular weight = 1960 ( = 33.5)

In a 2000-litre reactor previously neutralised with anhydrous nitrogen, 75 kg of dimethylpropionic acid (558.8 mol) are dispersed in 100 kg of cellosolve acetate (ethylene glycol acetate) at 70-80 ºC, then 200 kg of polyester melt (A) at 70 ºC are added together with 800 kg of liquid polyester (B), which corresponds to 98.4 mol of (A) and 408.2 mol of (B).

The mixture is heated to a total temperature of 75 ºC with vigorous stirring, while at the same time the medium is kept neutral by a flow of anhydrous nitrogen at 0.3 Nm³/h and the heating is regulated so that a temperature of 75±30 ºC is maintained.

558.2 kg of 4,4'-diisocyandicyclohexylmethane (2130 mol) is then added and the temperature is reduced to about 65 ºC; soon after, 0.3 kg of dibutyltin dilaurate is added and the temperature is allowed to rise until the reaction is completely exothermic, cooling is only used when it exceeds 110 ºC.

A temperature in the range of 100-110 ºC is maintained for 3 h while 0.3 kg of dibutyltin dilaurate is added at intervals of 30 min up to a total consumption of 1.5 kg of this catalyst.

After slow cooling (for about 4 hours) to a temperature of 55-60 ºC, the isocyanate groups are titrated. An NCO value of 5.34% (compared to a theoretical value of 5.47%) and a molecular weight in the range of 1500-1650 are determined; the calculated amount of 24% hydrazine monohydrate solution necessary for proper polymerization is found to be equivalent to 216.6 kg.

In a 5000-liter reactor equipped with a double spindle for strong stirring, 2223 kg of deionized water, 17 kg of dispersant (oxyethylated nonylphenol with 10 mol of ethylene oxide) and 67.7 kg of triethylamine are added.

The isocyanate prepolymer solution at 55-60 ºC is added to this aqueous solution.

The addition is carried out while stirring the aqueous solution vigorously and trying not to exceed 30 ºC.

After complete addition of the prepolymer solution, 216.6 kg of a 24% aqueous solution of hydrazine monohydrate is added.

Stirring is maintained for about 30 minutes and the emulsion thus prepared is now ready for use.

Example 2

Using the same equipment as in Example 1 and under identical conditions, a prepolymer is prepared by mixing 1039 kg of polybutylene glycol (H-[O-(CH₂)₄] -OH with = 26.13) with a number average molecular weight of 1900, 144 kg of N-methylpyrrolidone and 78.2 kg of dimethylolpropionic acid.

The temperature is raised to 70 ºC and 380.4 kg of isophorone diisocyanate and 0.36 kg of dibutyltin dilaurate are then added.

Allow the exothermic reaction to proceed and proceed as in Example 1.

The isocyanate groups are titrated and a value of 2.77% (compared to a theoretical value of 2.86%) for NCO is obtained, as well as a molecular weight in the range of 2500-2700. The stoichiometric hydrazine monohydrate is calculated to be 110.7 kg.

The aqueous solution is prepared as in Example 1 by dissolving 20 kg of dispersant (oxyethylated nonylphenol with 10 mol of ethylene oxide) and 60 kg of dimethylethanolamine in 2033 kg of water.

To proceed further, the prepolymer solution is added to the aqueous solution as described in Example 1, and 110.7 kg of a 24% hydrazine monohydrate solution is then added. This gives a polyurethane emulsion which has a solids content of approximately 38% and a pH in the range of 9.

This emulsion contains the already completed polyurethane and is ready for use in a hide upgrading process.

Example 3

Using the same equipment and conditions as in Example 1, a prepolymer is prepared by mixing 678.4 kg of polypropylene glycol

with = 33.4) with a number average molecular weight of 1958 with 120.6 kg of N-methylpyrrolidone, 22.2 kg of a polyethylene glycol/polypropylene glycol block copolymer (with a number average molecular weight of 2000 and an ethylene oxide content of 10 mol%) and 49.9 kg of dimethylpropionic acid.

This mixture is heated to 70 ºC and a first quantity of 114.7 kg of 4,4'-diisocyandicyclohexylmethane and 0.13 kg of dibutyltin dilaurate is added. The isothermal reaction is allowed to proceed and, after this has raised the temperature to about 85 ºC, the temperature begins to drop. The second quantity of 229.4 kg of dicyclohexylmethane diisocyanate and 0.13 kg of dibutyltin dilaurate is then added.

If the exothermic reaction is allowed to proceed, the temperature reaches about 110 ºC.

Amounts of 0.13 kg of dibutyltin dilaurate are added at 30-minute intervals to a total of 0.65 kg of catalyst. The mixture is then allowed to cool to a temperature of 60 ºC over a period of about 5 hours, and the prepolymer is then ready for emulsification (NCO = 3.31%; MW = 2200-2350).

The aqueous solution is prepared according to the procedure in Example 1 with 1482 kg of demineralized water, 15.8 kg of dispersant and 62.0 kg of dimethylethanolamine.

The prepolymer is emulsified in this aqueous solution, and then 98.0 kg of 24% aqueous hydrazine monohydrate is added.

The resulting emulsion has a dry matter content of 35% and a pH value in the range of 9-10.

According to the drawing, a web 1 of release paper coming off a drum 2 is pretreated with a solution of a conventional protective release agent as it passes through a pretreatment station 3 and then dried in an oven such as a gas-fired oven 4. On leaving the oven 4, a 0.05-1 mm thin layer of the aqueous polyurethane emulsion according to Example 3 above is applied to the release paper web. This application operation is preferably carried out by a station 4 using a doctor blade 6 so that the thin thickness desired for this layer can be controlled and adjusted. On leaving the station 5, the release paper web is dried as it passes through an oven 7 which may be gas-fired and the internal temperature of which is controlled in the range 70-90 ºC. On leaving this oven 7, a second layer of the same aqueous polyurethane emulsion is applied to the previously dried first polyurethane layer. This application operation is carried out in a booth 8 under Application of a spraying process with airless guns and feed pumps.

On leaving the booth 8, a split leather 9 to be upgraded is placed on the layer of aqueous polyurethane emulsion (which essentially forms a binding agent) and pressed. The pressing takes place with the interposition of a protective paper film with a pair of pressure rollers 10, 11, which are subjected to an adjustable pressure. After the split leather has been pressurized, the release paper web is passed through a final oven 13 in which the last drying step is carried out, the oven being heated to a temperature that can be regulated in the range of 70-90 ºC. On leaving the oven 13, the upgraded split leather is obtained by pulling off the release paper web underneath, which in turn is wound up on a separately arranged drum 14.

After very rigorous testing at the exit of the drying oven for the first polyurethane layer and on the upgraded final product, there are no pockmarks, cracks or any other signs of discontinuity in the coating. The upgraded split leather shows a flawless finish and the thickness of the polyurethane layer is in the range of 0.08-0.1 mm.

After examination by tanning experts, it was found that split leather enhanced by the process according to the invention was comparable and quite similar to naturally grained hides, but could not be confused with them.

It should be noted that the unexpected results given above were obtained with a feed rate of web 1 in the order of 6 m/min.

It should also be noted that the facility for carrying out the process according to the invention had a total length of 50 m, with the drying ovens having a total length of 28 m.

Using the same equipment as described above and identical process steps, the aqueous polyurethane emulsion of Example 2 was used to upgrade split leather and the same results were obtained both both in terms of the quality of the upgraded product and from a technical point of view.

To the aqueous polyurethane emulsion of Example 2 was added a glycol ether in an amount between 7 and 13 wt.%, based on the weight of water, to obtain essentially an azeotropic emulsion of this same polyurethane.

Using the above-mentioned device, the process according to the invention can be carried out by increasing the operating temperatures of the various ovens to values in the range of 90-120 ºC, so that the feed rate of the release paper web can be increased to 8 m/min.

The resulting increase in the hourly production rate of upgraded split leather has also shown in the latter case none of the defects occurring in the state of the art.

Comparable excellent results were obtained when the molecular weight of the prepolymer was changed from 1500 to 3200.

By reacting a polyether with the formula with = 33.7,

a polyesterdiol with the formula

H-[O-(CH₂)₄- O -CO-(CH₂)₄-CO] -OH

with = 10.2,

a carboxylated diol with the formula

a diisocyanate such as 4,4'-diisocyandicyclohexylmethane using the same procedure and percentages as indicated in Example 1, an isocyanate prepolymer can be obtained which, after emulsification according to the procedure described in Example 1 and polymerization to the highest possible molecular weight, gives a polyurethane emulsion with which, after application as indicated above, using the system indicated above and adding about 10% glycol ether, based on the weight of water, it was possible to increase the temperatures of the drying ovens up to 130-135 ºC and to improve the throughput up to a web feed rate of 10 m/min, without causing surface cracks or the formation of pockmarks.

Split leather processed according to the method of the invention consistently exhibits properties comparable to those of a naturally grained hide, as well as the ability to accommodate different textures of hides with different surface finishes, such as naturally grained ox hide and goat skin, paddling, ruffled and fantasy patterns. From a mechanical point of view, split leather upgraded according to the method of the invention exhibits unusual strength in terms of both flexural strength and dry and wet strength and high abrasion resistance, so that it can be used advantageously and reliably for the manufacture of leather goods, traditional shoes, sports shoes, etc. The thin (0.05-0.1 mm) upgrade layer enables the split leather to be sheared in exactly the same way as naturally grained or full-grain leather. As a result, split leathers that have been upgraded using the process according to the invention have a high industrial value, even though they are a by-product of the tanning industry.

It should also not be overlooked that the use of water-based polyurethane emulsion ensures environmental protection.

Claims (6)

1. Process for dressing hides, leather and similar fibrous substrates for upgrading split leather according to the so-called transfer process, which uses a supporting fabric of release paper onto which a thin layer of an aqueous polyurethane emulsion is applied, the polyurethane of the emulsion being obtained from an isocyanate prepolymer with a glycol repeat unit, characterized in that the isocyanate prepolymer has an average molecular weight in the range of 1500 to 3200 and that the glycol repeat unit contains at least one acidic polar group. 2. Process according to claim 1, characterized in that the polyurethane concentration in the aqueous solution is in the range of 25 to 50% by weight. 3. Process according to claim 2, characterized in that 5 to 15 wt.% glycol ether, based on the weight of water, is added to the aqueous solution to obtain a water/glycol ether azeotrope. 4. A fibrous substrate such as hide, leather, split leather and the like, which is finished and enhanced according to the process of claims 1 to 3. 5. Aqueous polyurethane emulsion for dressing skin, leather and similar fibrous substrates and for Upgrading of split leather, in which the polyurethane is obtained from an isocyanate prepolymer, characterized in that the isocyanate prepolymer has an average molecular weight in the range of 1500 to 3200 and that the polyurethane is contained in the emulsion in a proportion of 25 to 50 wt.%. 6. Aqueous polyurethane emulsion according to claim 5, characterized in that it has a water/glycol ether azeotrope.